1. Field of the Invention
The invention relates to electric-arc furnace electrodes and, more particularly, to an electrode assembly of the type including abutting carbon or graphite electrode sections which are securely held in end-to-end relationship by a pin threaded into a socket in the end of each electrode section.
2. Description of the Prior Art
Electric-arc furnaces are well known in the steel-making and metal-smelting industries. In the operation of an electric-arc furnace, an electric arc is established between an electrode and a conductive charge of material in the furnace to melt the furnace charge. Electrode systems in arc furnaces include an electrode, usually formed by a large rod-like member which projects downwardly into the furnace, and a movable electrode support structure, formed by a mast and an electrode holder, located outside the furnace. A drive mechanism lowers and raises the electrode support structure to move the electrode toward and away from the furnace charge. The electrode is connected to an electrical power supply which provides power for establishing the arc.
An arc is established as the electrode is moved toward the charge. After the arc is established, it is stabilized and maintained by controlling the position of the electrode relative to the charge in response to sensed arc conditions. Drive mechanism control circuitry has been employed for governing positioning of the electrode relative to the furnace charge in response to sensed arc conditions after an arc has been established.
Electrodes used in arc furnaces are of two basic types which commonly are referred to as "consumable" and "nonconsumable". Consumable electrodes generally consist of a metallic material which is melted to form a part of the furnace charge. Consumable electrodes are structurally more durable than the nonconsumable electrodes, but these electrodes can also be broken or otherwise damaged when driven into engagement with the furnace charge.
The present disclosure is directed to so-called non-consumable electrodes usually constructed or connected, rod-like sections which are not consumed in the sense that they do not form a part of the furnace charge. They actually are consumed relatively slowly during use due to arc erosion and oxidation. Typically the materials used as nonconsumable electrodes comprise carbon, graphite, or a similar carbonaceous material which both (a) conducts electricity and (b) is suitable to establish an arc with the furnace charge. As the electrodes are consumed, the remaining portion of the electrode must be advanced gradually toward the material being heated to maintain an appropriate arc. Additional electrode sections must be added one-by-one as the electrode extending into the furnace is advanced and consumed.
In order to join the electrode sections in end-to-end, abutting relationship so that they may be advanced serially into the furnace, each electrode section includes a threaded socket at each end. The threaded sockets extend longitudinally of the electrode and each is adapted to receive a threaded pin. The electrode sections may be joined by first inserting a pin into one of the sockets of the new electrode section to be added and then threading this assembly onto the exposed socket of the last electrode section.
It is desirable that the pins and the electrode sections be of similar material and that the material be rather homogeneous. This is true because (a) the electrode should have consistant current-carrying and arc-establishing characteristics and (b) the pin material, like the electrode should produce gaseous products when consumed so that the furnace charge will not be affected. These carbonaceous materials used suffer from the drawback that their mechanical properties are not well-suited to stresses caused by bending loads. These carbonaceous materials undergo very little elastic and plastic deformations when stressed. Consequently, cracks can be initiated very easily under repeated loads.
Breakage of nonconsumable electrodes has become a serious problem in the industry not only because of the direct costs incurred as a result of breaking the electrodes themselves, but also as a result of consequent production losses and repair and replacement costs.
Electric arc furnaces are normally "three-phase" furnaces in that each furnace includes three separate electrode systems. When an electrode is broken, it normally leaves a large broken-off portion in the furnace. The broken-off portion may or may not be salvagable, but in any event furnace operation must be terminated to enable retrieval of the broken-off portion and replacement of it. Extended idle time of a furnace for repair and replacement of broken electrode results in substantial production losses as well as exposing unbroken electrodes to excessive oxidation.
In the past, one major cause of breakage has occured when a melting operation is commenced. As the operation begins, drive motor controls operate the drive motor to advance the electrode toward the furnace charge. When an arc fails to be properly established as the electrode approaches the furnace charge, the electrode drive motor will drive the electrode into the charge. In the absence of drive motor controls responsive to contact of the furnace charge by the electrode, damage to the electrode is virtually inevitable. This is especially true when the surface struck by the electrode is at an angle with respect to the horizontal, with the result that lateral forces are applied to the electrode.
An improved apparatus for protecting electrodes from damage is taught by U.S. Pat. No. 3,937,869. Force-sensing circuitry monitors forces applied to an electrode system as it is positioned with respect to the furnace. Force-responsive controls inhibit electrode movement in response to predetermined rates of changes in sensed reaction forces during positioning of the electrode. Compensating circuitry continuously compensates for changes in forces arising from electrode weight variations and position changes of the electrode support structure so that the sensitivity of the control apparatus to applied forces which can damage the electrode system remains consistently high.
Although the breakage rate of electrodes in electric-arc furnaces has been reduced, breakage still occurs, especially in furnaces which are not equipped with the system of the referenced patent. Breakage also occurs due to problems which are not solved by the system of the referenced patent.
A particular type of breakage which remains unresolved is that resulting from caving-in of the scrap. When an electrode is lowered during the melting process, high mounds of unmelted scrap may be left surrounding the electrode. Under these conditions melting may reach a stage where the base of the scrap mound becomes unstable, and chunks of scrap tumble down and strike the sides of the electrode column. If the electrodes are comprised of a carbonaceous material, the flexural strength of the electrode column is low, and the scrap cave-in process often results in a broken electrode column.
A failure caused by a bending load on the column usually will occur at the section of highest bending moment and at a location where the stresses are highest. The uppermost electrode socket is subjected to larger bending moments than lower sockets; consequently, this is where the failure usually occurs.
An early electrode socket is shown in Broadwell, U.S. Pat No. 1,510,134, which discloses a straight-sided pin, rounded threads, and a clearance between the pin and the bottom of the socket to prevent "wedging strains" at the bottom of the socket. The pin of Broadwell is tapered slightly at its ends to facilitate coupling with the socket and the threads are rounded, to "afford the maximum contacting surface for electrical conduction."
Later patentees recognized that a barrel-shaped pin, that is, one tapered from its center to its ends, produced better results than the straight-sided pin of Broadwell. The thread designs of the pin and socket also were changed considerably from the rounded thread design of Broadwell. Representative designs are shown by the patents to Stroup, U.S. Pat. No. 2,836,806, and Kaufmann et al, U.S. Pat. No. 2,957,716, which designs failed to reduce breakage to acceptable levels.
Stieber et al, U.S. Pat. No. 3,612,586, theorized that the most important factor in strengthening a connection was not so much whether a straight-sided pin or a barrel-shaped pin was used, but whether the thread loading nearest the end faces of the electrodes exceeded a certain limit. In order to relieve the stresses supposedly concentrated at the flank portion of the threads nearest the electrode end faces, the socket threads simply were thinned or removed in this area so that the stresses would be shifted elsewhere. This approach, like those preceding, failed to yield acceptable results.
Lewis, U.S. Pat. No. 3,646,240, employed features of various prior art inventions as well as a different approach. Lewis utilized a barrel-shaped pin having a portion of the threads removed near the engaged faces of the electrode sections, somewhat rounded threads, and a bulbous counterbore forming the base of the socket. Lewis stated he "prevents the build up of destructive stress concentrations, such as thread roots at the base of the electrode socket". Lewis's approach employed features of prior art electrode socket designs with the addition of the voluminous counterbores "to transfer the load from electrode proper to joint threads and back again without destructive concentration of stress." The counterbores in Lewis intentionally were made exceedingly large to provide portions of the electrode less stiff than the electrode proper and thereby serve as flexible transition zones between the electrode proper and the connecting pin.
The primary shortcoming in Lewis's design stems from not removing the threads at the bottom of the socket, thus maintaining the stress concentration problem. Another problem associated with lewis's design results from the extreme size of the counterbore. Because the counterbore is so large, the wall thickness of the electrode section is reduced to the point where failure most likely will occur in the area of the counterbore. This is aggravated when the outer surface of the electrode has been attacked by oxidation. Hence, although Lewis recognized the base of the socket as a problem area, he did not solve the problem but only shifted it to another portion of the electrode section.
A more recent development in this area is shown by the patent to Kozak, U.S. Pat. No. 3,708,601. Kozak, like Lewis, recognized that a stress concentration area is the base of the socket and attempted to relieve stresses in this area. Kozak simply removed the last several threads near the base of the socket so that the connecting pin engaged all of the threads of the socket and extended slightly beyond the last socket thread without engaging the base of the socket. As a result of the thread removal, the notch angle between the last engaged socket thread and the socket wall is increased. According to Kozak, tests of his thread relief show reduced stress concentration on the order of thirteen percent compared to the designs discussed earlier. But, Kozak's structure fails to increase the socket strength sufficiently to satisfy the requirements of present-day industry.
While these and other attempts have been made to strengthen electrode section joints, the persistence of the problem itself simply demonstrates the shortcomings of those attempts. The physical properties of carbonaceous materials have seemingly not been fully understood at least from the standpoint of the configuration of mechanical components to provide section joints of maximized strength. While others have apparently recognized that excessive breakage occurs near socket bases, no one has found an appropriate solution of the problem.